Surface forces and contact angles

Karisa Steffens, Tech Editor

At 4:00 p.m. on Monday, Feb. 3, Lei Pan will be on campus to give a presentation about his research on the interaction of forces between water droplets and surfaces. His presentation will be given in room 610 of the Minerals and Materials Engineering Building on campus, and will last for about an hour.

Surfaces that are placed outside are often influenced by the forces of nature. However, research has been constantly conducted to observe exactly how the forces of water interacts with these surfaces. For each surface, the reaction is different. Some surfaces absorb water, or in other words, are hydrophilic. As a term that literally translates as “water loving” it is quite a fitting word. Others, however, repel water. These surfaces are considered hydrophobic, and are the surfaces we recognize as ones on which water will form droplets on top of, instead of soaking into the material, like sponge.

This isn’t a mysterious phenomenon to anyone watching; most people have seen water behave this way and are aware that different materials react differently to being placed in water. Yet, there are still more underlying factors for when an object is considered wet. In technical terms, when the air film between a water droplet and a surface is ruptured, it would be called wetting the surface. Many do not understand when this occurs, though it is a well-known phenomenon. Each water droplet on a surface forms a very specific and finite contact angle when it beads on top of the surface. Pan’s research group has been focusing on studying this, and the forces between these water droplets and surfaces. For quite some time, this interaction has not been well understood, and this was why this research group sought to know more about it, and develop methods to model and understand these interactions in the future.

Pan’s research group developed what is known as a synchronized tri-wavelength reflection interferometry microscopy technique to characterize the instability of air films to start. This technique systematically measures the thickness of air films between droplets of water and surfaces over time through use of a special microscope known as a synchronized tri-wavelength reflection interferometry microscope. By understanding this thickness for different surfaces, conclusions can be drawn about when the air film would rupture.

Pan will also be discussing a numerical method his group established to determine the surface forces between a liquid droplet and a surface. This method along with their research has helped to reveal strongly attractive electrostatic forces between water droplets and hydrophilic surfaces across air films. After discussing his findings within this research, Pan will talk about applications of what this research can do today.

Lei Pan is an assistant professor of chemical engineering at Michigan Technological University. His lecture will be open to all students and faculty on campus. For anyone interested in understanding more about these new discoveries about forces between water droplets and surfaces, this would be the chance to learn more.